System and methods for automatically re-signaturing multi-unit data storage volumes in distributed data storage systems

ABSTRACT

Replicated data storage units are autonomously identified and assembled into generationally related data storage volumes. A data storage manager, implementing a re-signaturing process executed at defined intervals or manually initiated on a server or client system connected to the storage area network, scans the collection of visible data storage units to identify those related as a data storage volume. Each replicated data storage unit includes metadata that embeds an identification of the replicated data storage unit and volume accessible to the data storage manager. To assemble a set of replicated data storage units into a generational volume, the data storage unit metadata is rewritten to establish a unique data storage volume identity including information to associate the data storage volume in a lineage with the source data storage volume.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to the management and controlof distributed virtual storage systems and, in particular, to themanagement and re-signaturing of data storage units within distributedvirtual storage systems.

2. Description of the Related Art

As computer systems scale to enterprise levels, particularly in thecontext of supporting large-scale data centers, the underlying datastorage systems frequently adopt the use of storage area networks(SANs). As is conventionally well appreciated, SANs provide a number oftechnical capabilities and operational benefits, fundamentally includingvirtualization of data storage devices, redundancy of physical deviceswith transparent fault-tolerant fail-over and fail-safe controls,geographically distributed and replicated storage, and centralizedoversight and storage configuration management decoupled fromclient-centric computer systems management.

Architecturally, a SAN storage subsystem is characteristicallyimplemented as a large array of Small Computer System Interface (SCSI)protocol-based storage devices. One or more physical SCSI controllersoperate as the externally accessible targets for data storage commandsand data transfer operations. The target controllers internally supportbus connections to the data storage devices, identified as logical units(LUNs). The storage array is collectively managed internally by astorage system manager to virtualize the physical data storage devices.That is, the SCSI storage devices are internally routed and respond tothe virtual storage system manager as functionally the sole hostinitiator accessing the SCSI device array. The virtual storage systemmanager is thus able to aggregate the physical devices present in thestorage array into one or more logical storage containers. Virtualizedsegments of these containers can then be allocated by the virtualstorage system as externally visible and accessible LUNs with uniquelyidentifiable target identifiers. A SAN storage subsystem thus presentsthe appearance of simply constituting a set of SCSI targets hostingrespective sets of LUNs. While specific storage system managerimplementation details differ as between different SAN storage devicemanufacturers, the desired consistent result is that the externallyvisible SAN targets and LUNs fully implement the expected SCSI semanticsnecessary to respond to and complete initiated transactions against themanaged container.

A SAN storage subsystem is typically accessed by a server computersystem implementing a physical host bus adapter (HBA) that connects tothe SAN through network connections. Within the server, above the hostbus adapter, storage access abstractions are characteristicallyimplemented through a series of software layers, beginning with alow-level SCSI driver layer and ending in a operating system specificfilesystem layer. The driver layer, which enables basic access to thetarget ports and LUNs, is typically vendor specific to theimplementation of the SAN storage subsystem. A data access layer may beimplemented above the device driver to support multipath consolidationof the LUNs visible through the host bus adapter and other data assesscontrol and management functions. A logical volume manager (LVM),typically implemented intermediate between the driver and conventionaloperating system filesystem layers, supports volume orientedvirtualization and management of the LUNs accessible through the hostbus adapter. Multiple LUNs can be gathered and managed together as avolume under the control of the logical volume manager for presentationto and use by the filesystem layer as an integral LUN.

In typical implementation, SAN systems connect with upper-tiers ofclient and server computer systems through a communications matrixfrequently implemented using a Fibre Channel (FC) based communicationsnetwork. Logically, a Fibre Channel network is a bidirectional,full-duplex, point-to-point, serial data channel structured specificallyfor high performance data communication. Physically, the Fibre Channelis an interconnection of multiple communication ports, called N_Ports,implemented by the host bus adapters and target controllers. Thesecommunication ports are interconnected by a switching network deployedas a n-way fabric, a set of point-to-point links, or as an arbitratedloop.

Strictly defined, Fibre Channel is a generalized transport mechanismthat has no high-level data flow protocol of its own or nativeinput/output command set. While a wide variety of existing Upper LevelProtocols (ULPs) can be implemented on Fibre Channel, the mostfrequently implemented is the SCSI protocol. The SCSI Fibre ChannelProtocol (FCP) standard defines a Fibre Channel mapping layer thatenables transmission of SCSI command, data, and status informationbetween a source host bus adapter, acting as a SCSI initiator, and adestination SCSI target controller, over any Fibre Channel connectionpath as specified by a Fibre Channel path identifier. As definedrelative to a target, a FC path identifier is a reference to thedestination port and logical unit of the SAN storage system. The port isuniquely specified by a World Wide Port Name (WWPN). The LUN identifieris a unique, hardware independent SCSI protocol compliant identifiervalue retrievable in response to a standard SCSI Inquiry command.

A common alternative transport mechanism to Fibre Channel is defined bythe Internet Small Computer System Interface (iSCSI) standard. Insteadof relying on a new FC media infrastructure, the iSCSI standard isdesigned to leverage existing TCP/IP networks including specifically theexisting mixed-media infrastructure, including typical intranet andinternet networks, and to use internet protocol (IP) layer forupper-level command and data transport. Unlike Fibre Channel, the SCSIprotocol is the exclusive upper-level protocol supported by iSCSI. Thatis, the iSCSI protocol semantics (IETF Internet Draftdraft-ietf-ips-iSCSI-08.txt; www.ietf.org) specifically requires thetransmission of SCSI command, data, and status information between SCSIinitiators and SCSI targets over an IP network. Similar to the FC path,an iSCSI path, as specified by a SCSI initiator, is a combination of atarget IP address and LUN identifier.

SAN virtualization of client LUNs enables a number of specificcapabilities, including a more efficient use of the storage space withina particular container, dynamic extensibility and reconfiguration of thecontainer storage space by adding and replacing physical devices andshifting unused storage space between localized containers, andcomprehensive management of the virtual LUNs. In addition, modern SANsystems enable multiple network path (multipath) access between the SANconnected computer systems and multiple, different physical SAN storagesystems. Multipath routing functionally enables configuration ofredundant network connections and channel bonding to achieve fundamentalincreases in the total available bandwidth between clients and theirdata stores.

A particular benefit of conventional SAN systems is the ability toimplement consistent, system-oriented data integrity protectionpolicies. Given the scope of data stored by individual SANs, overallstorage system reliability and ongoing data integrity are baselinerequirements. To provide the various real-time, hot-backup, and similarcapabilities of conventional SAN systems, these systems will typicallyimplement a periodic or administratively driven data replication-baseddata integrity protection scheme. Persistently scheduled checkpointevents are typically used to initiate image replication, also referredto as snap-shot copy, of established, externally visible LUNs. Subjectto the details of the various sparse and progressive data copytechniques that may be used by any particular proprietary SANimplementation, each checkpoint event drives the creation ofpoint-in-time copies of the event-specified externally visible LUNs.

Administratively, checkpoints will be set to encompass full clientcomputer system volumes in order to preserve potential internal datadependencies between the LUNs that make up individual volumes. LUNreplication services, as implemented by SAN systems, conventionallyexecutes independent of volume identification; snap-shot copies are madeof individual, externally visible LUNs without regard to volumeparticipation. To prevent logical identification collisions between thesource LUNs and replicated LUN copies, the check-point LUN copies aremarked inactive integral to the copy process. Thus, beyond possiblyinitial administrative identification of LUNs for replication, there islittle required administrator intervention and essentially no uservisible burden arising from the direct execution of LUN replicationoperations.

Although a generally infrequent requirement, various circumstances mayrequire a rollback of a client computer system volume to a priorpoint-in-time. In other circumstances, a user may wish to haveconcurrent access to both a prior checkpointed instance and the currentinstance of a volume. Unfortunately, restoring a checkpointed volume orsimply making a checkpointed volume currently accessible is typicallyburdensome. Conventionally, a manual selection operation is required toparse through all of the prior replicated LUNs to select just the singleconsistent set of LUNs that represent a specific, desired point-in-timereplicated volume. Given the multiplicity of checkpoint events, thenumber of LUNs under management by a SAN system, and the number of SANsystems within the use scope of a given server computer system, theprocess of identifying, validating, restoring, and mounting a priorreplicated volume set of LUNs is complex and time consuming. Moreover,manually managed volume restoration presents a significant risk that thedata integrity of the restored volume will not be maintained. If theintegrity of a restored volume is lost, then the integrity of one if notmultiple other replicated generations of volumes will be corrupted.

Even where a proper set of replicated LUNs are identified forrestoration, restoring a prior copy of a current volume is complicatedby the inherent duplicate nature of the replicated LUNs. As expected,the internal data structures of the replicated LUNs, including the LUNinternal metadata, are preserved by the replication process. Remountinga prior point-in-time copy of a volume can create an identity ambiguitysince the replicated LUNs will report their original LUN identifiers.Furthermore, since there is no default defined manner of handling theLUNs of replicated volumes in a SAN environment, different clientcomputer systems may operate inconsistently in recognizing the identityof the LUNs that constitute a currently active volume. Inconsistentrecognition of LUNs is of particular concern given that current andreplicated LUNs are not guaranteed visible to all client computersystems at all times in a SAN environment. Even where a volume consistsof just a single LUN, different client computer system could fail todistinguish which is the currently active volume, leading toinconsistent use.

Client-based software and protocols to resolve these ambiguities createa further layer of complexity, are dependent on the mutual communicationbetween the client computer systems, and requires reliable connectivityto ensure continuous communication. Therefore, before a replicated LUNset can be mounted by any client computer system as an active volume,the replicated LUNs must be suitably re-signatured to ensure both LUNand volume uniqueness within the scope of any accessing client or servercomputer system. Conventional administrative tools do support therewriting of selected LUN identifiers and volume signatures as part ofthe manual process of selecting and remounting a prior point-in-timevolume. Still, this re-signaturing process is an additional and requiredstep in the already complicated manual process of enabling access to aprior point-in-time copy of a volume. Additional complexities arise inthe context of rollbacks where there is a need to repeatedly selectbetween and activate multiple replicated volumes, while ensuring thatthe original state of the client computer systems volumes can bemaintained and reliably restored.

Therefore, particularly as the use of SAN systems and reliance oninternal replication services grows, there is a present need for usersto be able to easily, reliably, and preferably transparently, manage andmaintain LUN volume sets to enable rollback and remounting ofcheckpointed data storage volumes.

SUMMARY OF THE INVENTION

Thus, a general purpose of the present invention is to provide anautonomous mechanism for recognizing and assembling data volumes fromdata storage units created automatically by the ongoing replicationservices provided by SAN storage devices.

This is achieved in the present invention by providing a system whereinreplicated data storage units are autonomously identified and assembledinto generationally related data storage volumes. A data storagemanager, implementing a re-signaturing process executed on a server orclient system connected to the storage area network, scans, atpredetermined intervals, the collection of visible data storage units toidentify those related as a data storage volume. Each replicated datastorage unit includes metadata that embeds an identification of thereplicated data storage unit and volume accessible to the data storagemanager. To assemble a set of replicated data storage units into agenerational volume, the data storage unit metadata is rewritten toestablish a unique data storage volume identity including information toassociate the data storage volume in a lineage with the source datastorage volume.

An advantage of the present invention is that replicated data volumesare made available through reassembly with a certainty of maintainingdata integrity both within the assembled data volume and as among thecollection of data storage units that may be visible to a clientcomputer system at any given time.

Another advantage of the present invention is that the assembly ofreplicated data storage units into volumes proceeds effectivelyindependent of the potentially proprietary replication processesimplemented by any particular storage area device. The re-signaturingprocess of the present invention is capable of proceeding on anindependent schedule that, relative to the SAN, only depends on thecurrent visibility of the necessary data storage units within the datastorage network to proceed with and complete reassembly.

A further advantage of the present invention is that once there-signaturing process is administratively established, the process ofre-signaturing replicated data storage units to support reassemblyproceeds autonomously with respect to both users and administrators. Thepresent invention provides for the autonomous re-signaturing ofreplicated data storage volumes into a corresponding generationallyrelated series of data storage volumes that are then available formounting or use in a roll-back operation.

Still another advantage of the present invention is that there-signaturing process is fully qualified based on the data present andaccessible from the data storage units of the replicated data storagevolumes. The decision of whether a given data storage unit needs to bere-signatured can be determined from the data storage unit itselfwithout reference to any other data storage unit.

Yet another advantage of the present invention is that there-signaturing process is self-deterministic in recognizing andprocessing volume related sets of data storage units. Instances of there-signaturing process run from different host computer systems againstthe same SAN storage systems will realize the same end result. There-signaturing process of the present invention is thus capable ofconcurrent, fully distributed execution without fault and therefore wellsuited for automated use with SANs and other storage systems supportingdistributed access to data storage volumes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an architectural system schematic illustrating an intendedoperating environment of a preferred embodiment of the presentinvention;

FIG. 2 is a block diagram of a SAN data storage device as implementedfor use in conjunction with a preferred embodiment of the presentinvention;

FIG. 3 is a block diagram detailing the association of multiple levelsof device metadata as implemented in a preferred embodiment of thepresent invention;

FIG. 4 is a block diagram illustrating the distributed filesystem andlogical volume management architecture utilized by a preferredembodiment of the present invention;

FIG. 5 is a detailed architectural block diagram illustrating a virtualmachine-based filesystem and logical volume manager as implemented in apreferred embodiment of the present invention.

FIG. 6 is a block diagram illustrating the potential association oflogical data storage volumes across multiple SAN storage devices and theprocess association that allows reassembly in accordance with apreferred embodiment of the present invention;

FIGS. 7A-C detail a preferred replication and reassembly progression asimplemented in a preferred embodiment of the present invention;

FIG. 8 details a more complicated replication and reassembly progressionas implemented in a preferred embodiment of the present invention;

FIG. 9 provides a state flow diagram that illustrates a process of datastorage unit replication as generally implemented in conjunction with apreferred embodiment of the present invention; and

FIG. 10 presents a state flow diagram illustrating the related processesof re-signaturing a data storage volume to support reassembly asimplemented in a preferred embodiment of the present invention.

DETAILED DESCRIPTION

The present invention is generally applicable in computing environmentswhere data storage volumes used by client computer systems are managedwithin a distributed storage system that supports typically automaticdata replication operations. Accordingly, a preferred environment forthe implementation of the present invention is in otherwise conventionalstorage area network (SAN) based data centers. From the followingdetailed description of the invention, however, those of ordinary skillin the art will readily understand that the present invention is notconstrained to use in a particular environment, system or networkarchitecture or by use of a particular operating system or set of datacommunications protocols. The following description of the presentinvention is presented in the context of a data center application asillustrative of a preferred embodiment for clarity of presentation andexplanation. Also for clarity of description, as used in the followingdetailed description of the invention, like reference numerals are usedto designate like parts depicted in one or more of the figures.

As generally shown in FIG. 1, a preferred data center 10 hostedimplementation of the present invention typically supports one or moretiers of computer systems 12 that, directly as clients or as serversoperating indirectly on behalf of one or more upper tiers of client andserver computer systems, provides access to logical units of storagehosted by a SAN 14 and underlying data storage systems 16. The SAN 14can be implemented using any of a variety of technologies, thoughtypically using Fibre Channel or iSCSI technology. These technologiesallow construction of a redundant, failover and multipath capableinterconnection network, using for example redundant routers 18 andnetwork connections, that in turn ensures overall reliability. In atypical implementation, additional data management features areimplemented through logical volume managers and data access layersexecuted in a server tier of computer systems 12. Client computersystems are constrained to mounting and accessing data storage volumesthrough the server tier and thereby effectively inherit the logical unitmanagement functions implemented by the logical volume managers of theserver tier. Logical volume managers, however, can be and frequently areimplemented at multiple levels including in client computer systems.

The different potential locations of logical storage managers aregenerally not significant to the operation of the SAN 14 and, inparticular, the underlying data storage systems 16. While the SAN 14provides routeable multipath access, the data storage systems 16presents a relatively large collection of externally visible LUNs, alsoreferred to in the context of the present invention as data storageunits (DSUs), accessible by the computer systems 12, subject toconventional access controls. Individually, the data storage systems 16are relatively conventional computer platforms 20, though specialized tosupport typically high-bandwidth fibre channel network interfaces and tohost large parallel arrays of typically SCSI-based disk drive storageunits 221-N. Aggregate network bandwidth at the SAN 14 interfacetypically in excess of 200 Megabytes per second and online storagecapacity in excess of 10 terabytes on a single system 16 is presentlynot uncommon. Collectively, the data storage systems 16 are oftengeographically distributed to reduce access latency, distribute load,and ensure that power and network disruptions do not compromise theentire function of the system 10.

Conventionally, a storage system manager 24 is executed on the storagesystem platform 20 to implement a virtualization of the physical,typically disk drive-based storage units 221-N present in the localstorage system 16. The storage system manager 24 performs the real tovirtual translations necessary to support the presentation of datastorage units to the computer systems 12 for use as, in effect, standardSCSI-based LUNs. This virtualization of the internal LUN storage allowsa more efficient utilization of the physical storage units 221-N throughlogical aggregation into a contiguous container storage space. Thecontainer may be dynamically reconfigured and expanded depending ondemand patterns without materially affecting the ongoing use of aparticular data storage system 16 by the computer systems 12; thepresentation of the data storage units can be preserved even whilemaintenance is performed on an array of physical storage units 221-N.

FIG. 2 provides a logical view 30 of a data storage system 16implemented in accordance with a preferred embodiment of the presentinvention. As viewed from outside of the SAN 14, the data storageplatform 18 logically presents an array of LUNs 321-N, as data storageunits, accessible through a network accessible target controller adapter34. Consistent with conventional SCSI LUN devices, each of the LUNs321-N internally store device and LUN metadata in a vendor specificmanner, which may be accessed using standard SCSI commands, such as SCSIInquiry to retrieve the LUN identifier. Functionally, for purposes ofdiscussion, the device and LUN metadata can be described as beingcontained in a device metadata table 36 and a LUN metadata table 38. Theset of LUN metadata is also vendor specific with an information contentappropriately defined and maintained, in the present context, thestorage system manager 24.

As further detailed in FIG. 3, the device metadata table 36 storesinformation that defines the structure of the virtualized device. Thisinformation includes the command level protocol that is recognized bythe virtualized device, vendor identification data, and data describingthe data storage space geometry including details such as block size anddefect extents and replacements. The device metadata table 36 alsofunctionally includes a reference to the LUN metadata table 38. Theprincipal data stored by the LUN metadata table 38 is the LUNidentifier. For SCSI-3 devices, the LUN identifier is a multi-byte valuethat is automatically generated and stored to the LUN metadata table 38as part of the initial LUN provisioning procedures. The LUN metadatatable 38 also maintains a typically table-based set of indirectreference values that functionally identify the location of the datastorage space 40 within the LUN 321-N.

In accordance with the present invention, a data storage volume (DSV)metadata table 42 is established within the data storage space 40 inconjunction with an otherwise conventional filesystem space 44.Following the provisioning of a particular LUN 321-N, filesystem controlstructures are typically initialized within the filesystem space 44. TheDSV metadata table 42 is preferably established outside of the logicaladdress space of the filesystem space 44, though at a known offset fromthe beginning of the data storage space 40. Accordingly, the location ofthe DSV metadata table 42 is determinable in relation to the LUNmetadata table 38, as generally indicated in FIG. 3.

A DSV metadata table 42, constructed in accordance with the presentinvention and as shown in FIG. 3, preferably contains the followingvalues:

DSU.ID:Nominally a copy of the LUN identifier value. Identifier of thecurrent data storage unit. Required. A multi-byte value consistent withthe definition of the SCSI LUN identifier value.

DSU.Num:Positional value identifying the ordering of this DSU within adata storage volume (DSV). Required. Preferably a 64-bit value.

DSV.ID:A globally unique identification value for the DSV. Required.Preferably a 128-bit value.

DSV.Card:Cardinality value identifying the number of DSUs contained inthis DSV. Optional. Preferably a 64-bit value.

DSV.Gen:Progressive value identifying the generation of this DSV.Required. Preferably a 64-bit value.

For the preferred embodiments of the present invention, the contents ofthe DSV metadata table 42 are populated and used through the executionof a re-signaturing management program preferably implemented as part ofor as an adjunct to a host-based logical volume manager (LVM).Consistent with the present invention, separate instances of there-signaturing management program can be independently executed among adistributed set of hosts, while fully and consistently achieving theresults of any one instance being executed, as described below.

As generally illustrated in FIG. 4, a typical system architecture 60implements a logical volume manager 62 on a computer system 12, that is,at a system tier above the data storage systems 16, as a software layerbeneath a local filesystem layer 64. By execution of the logical volumemanager 62, the filesystem layer 64 is presented with a data storageview represented by one or more discrete data storage volumes 66, eachof which is capable of containing a complete filesystem data structure.The specific form and format of the filesystem data structure isdetermined by the particular filesystem layer 64 employed. For thepreferred embodiments of the present invention, physical filesystems,including the New Technology filesystem (NTFS), the Unix filesystem(UFS), the VMware Virtual Machine filesystem (VMFS), and the Linux thirdextended filesystem (ext3FS), may be used as the filesystem layer 64.

As is conventional for logical volume managers, each of the data storagevolumes 66 is functionally constructed by the logical volume manager 62from an administratively defined set of one or more data storage unitsrepresenting LUNs. Where the LUN storage, at least relative to thelogical volume manager 62, is provided by network storage systems 16,the data storage volumes 66 are assembled from an identified set of thedata storage units externally presented by the network storage systems16. That is, the logical volume manager 62 is responsible forfunctionally managing and distributing data transfer operations to thevarious data storage units of particular target data storage volumes 66.The operation of the logical volume manager 62, like the operation ofthe storage system manager 24, is transparent to applications 68executed directly by computer systems 12 or by clients of computersystems 12.

A preferred system architecture 60, implementing a virtual machine basedsystem 70, is shown in FIG. 5. An integral computer system 72, generallycorresponding to one of the computer systems 12, is constructed on aconventional, typically server-class hardware platform 74, including inparticular host bus adapters 76 in addition to conventional platformprocessor, memory, and other standard peripheral components (notseparately shown). The server platform 74 is used to execute a virtualmachine (VMKernel) operating system 78 supporting a virtual machineexecution space 80 within which virtual machines (VMs) 821-N areexecuted. For the preferred embodiments of the present invention, thevirtual machine kernel 78 and virtual machines 821-N are implementedusing the ESX Server product manufactured and distributed by VMware,Inc., Palo Alto, Calif. Use of the ESX Server product and, further,implementation using a virtualized computer system 12 architecture, isnot required in the practice of the present invention.

In summary, the virtual machine operating system 78 provides thenecessary services and support to enable concurrent execution of thevirtual machines 821-N. In turn, each virtual machine 821-N implements avirtual hardware platform 84 that supports the execution of a guestoperating system 86 and one or more typically client applicationprograms 88. For the preferred embodiments of the present invention, theguest operating systems 86 are instances of Microsoft7 WindowsJ, LinuxJand NetwareJ-based operating systems. Other guest operating systems canbe equivalently used. In each instance, the guest operating system 86includes a native filesystem layer, typically either an NTFS or ext3FStype filesystem layer. These filesystem layers interface with thevirtual hardware platforms 84 to access, from the perspective of theguest operating systems 86, a data storage host bus adapter. In thepreferred implementation, the virtual hardware platforms 84 implementsvirtual host bus adapters 90 that provide the appearance of thenecessary system hardware support to enable execution of the guestoperating system 86 transparent to the virtualization of the systemhardware.

Filesystem calls initiated by the guest operating systems 86 toimplement filesystem-related data transfer and control operations areprocessed and passed through the virtual host bus adapter 90 to adjunctvirtual machine monitor (VMM) layers 921-N that implement the virtualsystem support necessary to coordinate operation with the virtualmachine kernel 78. In particular, a host bus emulator 94 functionallyenables the data transfer and control operations to be ultimately passedto the host bus adapters 76. The system calls implementing the datatransfer and control operations are passed to a virtual machinefilesystem (VMFS) 96 for coordinated implementation with respect to theongoing operation of all of the virtual machines 821-N. That is, thenative filesystems of the guest operating systems 86 perform command anddata transfer operations against virtual SCSI devices presenting LUNsvisible to the guest operating systems 86. These virtual SCSI devicesare based on emulated LUNs actually maintained as files resident withinthe storage space managed by the virtual machine filesystem 96. In thisrespect, the virtual machine filesystem 96 is to the virtual machines821-N what the storage system 16 is to the physical computer systems 12.Permitted guest operating system 86 command and data transfer operationsagainst the emulated LUNs are mapped between the LUNs visible to theguest operating systems 86 and the data storage volumes visible to thevirtual machine filesystem 96. A further mapping is, in turn, performedby a virtual machine kernel-based logical volume manager 62 to the LUNsvisible to the logical volume manager 62 through the data access layers98, including device drivers, and host bus adapters 76.

FIG. 6 illustrates the preferred operation of the present invention inthe autonomous recognition and reassembly 100 of data storage volumesarising from snapshot replication of LUNs by conventional networkstorage systems 16. In the example illustrated, two network storagesystems 16A,B are represented hosting corresponding sets of LUNs 32A,B.A defined set of the LUNs 32A, denoted 32A (M . . . N), are recognizedby the logical volume manager 62 as a first exemplary data storagevolume 102. A second defined set, denoted 32A (W), 32B (X . . . Y), arerecognized as constituting a second exemplary data storage volume 104that spans the network storage systems 16A,B. As is conventional, theindividual network storage systems 16A,B separately perform snapshotreplication of their LUN sets LUNs 32A,B. The replication instances ofthe LUNs identified as 32A (M . . . N, W) are created within the virtualstorage space of the network storage system 16A, while replicationinstances of the LUNs identified as 32B (X . . . Y) are created withinthe virtual storage space of the network storage system 16B.

In accordance with the preferred embodiments of the present invention, are-signaturing process 106 is executed as part of or in conjunction withthe logical volume manager 62. The re-signaturing process 106 preferablyexecutes at intervals to recognize replicated instances of LUNscorresponding to known data storage volumes 102, 104 and to autonomouslyperform re-signaturing of the replicated LUNs to enable reassembly ofdiscrete replication sets as generationally related data storage volumes102′, 104′. Once re-signatured, the data storage volumes 102′, 104′ willexist without any required dependency on the original replicated datastorage volumes 102, 104.

Recognition of the replicated instances is preferably performed by there-signaturing process 106 executing, at intervals, a bus scan againstthe network storage systems 16A,B visible to the logical volume manager62. The bus scan allows the re-signaturing process 106 to identify allof the data storage units available for consideration. Subsequent SCSIInquiry commands directed to the discovered data storage units isperformed by the re-signaturing process 106 to retrieve LUNidentification information. The information contained in the DSVmetadata tables 42 is also retrieved, using SCSI read commands, forexamination as needed to identify replicated data storage unitsrequiring re-signaturing. Once updated DSV metadata is determined by there-signaturing process 106, the updated metadata is written to the DSVmetadata tables 42 of the affected sets of data storage units using SCSIwrite commands. The logical volume manager 62 can thereafter referencethe updated sets of data storage units as generationally related datastorage volumes 102′, 104′. Preferably, atomicity in the re-signaturingprocess 106 for individual data storage units is ensured by guarding theread and write operations issued against a particular data storage unitwith SCSI Reserve and SCSI Release commands.

The re-signaturing and assembly of generationally related data storagevolumes 102′, 104′ is detailed in FIGS. 7A-C. Referring to FIG. 7A, anexemplary data storage volume 110 consists of three data storage units112, uniquely identified by LUN identifiers (LUN.IDs) AAAA, BBBB, CCCC.The DSV metadata uniformly identifies the data storage units 112 asbelonging to a unique data storage volume identified as XX and as anoriginal or zero generation instance of the data storage volume 110.

Snapshot replication implemented by the containing network storagesystem 16 produces a replicated set of data storage units including thereplicated instances 112′, as shown in FIG. 7B. In provisioning thereplicated instances 112′, new, unique LUN identifiers EEEE, FFFF, GGGGare automatically created and assigned to the data storage units 112′ asis conventional under the SCSI command protocol. The DSV metadata tables42 are copied without change to the replicated instance data storageunits 112′. At least by the inconsistency between the LUN identifiersand DSU.ID values maintained in the corresponding DSV metadata tables42, the replicated instance data storage units 112′ are autonomouslyidentifiable as requiring re-signaturing.

As reflected in the DSV metadata values shown in FIG. 7C, there-signaturing process 106 executes to update the DSV metadata tables 42in the replicated instance data storage units 112′ to render the datavalues consistent and permit reassembly as a data storage volume withoutconflict with the existing data storage units 112 of the zero generationdata storage volume. Specifically, the DSU.ID value is conformed, and aunique generation number is assigned. Optionally, a new unique volumeidentifier is generated, YY, and applied. Where a data storage volume isintended to be held inactive pending use in a rollback, the copied datastorage volume identifier DSV.ID is preferably not modified. Since,under rollback circumstances, the current and rollback target volumeswill not be active concurrently, but rather the intent of the rollbackis to identically restore state to the point in time of the rollbacktarget volume, maintaining the DSV.ID unchanged will allow directsubstitution of the rollback target volume for the current volumewithout concern for conflict.

Conversely, if reassembled data storage volumes are to be used toprovide concurrent access to current data and data current as ofdifferent replication points in time, then the storage volume identifierof appropriate sets of data storage units may be jointly updated tounique values. Of course, the DSV.ID value re-signaturing may bedeferred until such time as the volume is required for use. Depending onthe actual use desired, the DSV.ID value may be left unchanged orupdated to a unique value at the time the data storage volume isactivated by the logical volume manager 62. While, in generalapplication, the preference is to defer updating of the DSV.ID value,the value is shown as updated in FIG. 7C. The resulting data storagevolume 116 can then be recognized and managed by the logical volumemanager, including activation and mounting, without conflicting with theexisting zero generation data storage volume 110. In this case, the datastorage volume can and is preferably uniquely identified using the<DSV.ID, DSV.Gen> value tuple. Further, the resulting data storagevolume 116, once activated by the logical volume manager 62, can itselfbe subject to snapshot replication. That is, a generationally deriveddata storage volume 116, when re-deployed as a primary copy, can berecursively treated as a zero generation data storage volume 110 subjectto, in turn, replication and re-signaturing.

A more involved example of the operation 120 of the present invention isprovided in FIG. 8. A currently active, generation zero data storagevolume 122 is provided by the data storage systems 16. In response to acheckpoint event, snapshot copies 124 of the data storage unitscomprising the volume 122 are created. The data storage units 124 areidentical copies of the data storage units that comprise the generationzero data storage volume 122 with the exception of having unique LUNidentifiers. In response typically to a timer event, the re-signaturingprocess 106 proceeds to re-signature the LUNs 124 by conforming theDSU.ID values to match the LUN identifiers and incrementing thegeneration DSV.Gen to a unique value for the data storage volume.

In typical operation, the sequence of data storage units replication bythe data storage systems 16 and the operation of the re-signaturingprocess 106 is consistent and ordered. For example, replication may beadministratively set to occur at a defined time within each 24 hourperiod. The re-signaturing process 106 may be administratively set tooccur at one if not multiple times within each 24 hour period to ensurethat replicated data storage units are timely available for use. Typicalordered operation is generally represented by the ordered replicationand re-signaturing of the first and second generation data storagevolumes 124′ and 126′. The present invention, however, handles theout-of-order circumstance where multiple replications occur before are-signaturing process 106 is performed or, alternately, is able tocomplete.

As illustrated in FIG. 8, the operations giving rise to the third andfourth replication generations 128, 130 occur prior to the performanceof the re-signaturing process 106. The delay in performingre-signaturing may arise from a number of different causes, includingthe non-execution of the computer systems 12 that access the datastorage volume 120, fault conditions related to the logical volumemanager 62 that prevent appropriate access by the re-signaturing process106 to the data storage volume 120 and replicated data storage unitssets 128, 130, or a communications failure between the computer systems12 and data storage systems 16 that prevents concurrent access to theall of the data storage units required to assemble a particularreplicated data storage volume 128, which may more frequently occurwhere a part of the volume is spread across different physical datastorage systems 16, as illustrated in FIG. 6 in relation to the logicalvolume 104. As shown in FIG. 8, re-signaturing of the third generationdata storage volume 128′ proceeds after the re-signaturing of the fourthgeneration data storage volume 130′. To achieve autonomous operation,the present invention provides for recognition of the replicationcreation order of the data storage units as marked by the inception dateof the LUN provisioning. Additional verification of the completeness ofpotential data storage units sets is preferably recognized from thesequential series defined by the DSU.Num values and, where implemented,the DSV.Card value. Again, the re-signatured data storage volumegenerations 124′, 126′, 128′, 130′, once activated, are functionallyequivalent independently to the generation zero data storage volume 122with respect to being subject to autonomous replication andre-signaturing in accordance with the present invention.

A preferred process 140 for establishing the DSV metadata tables 42 inconnection with the provisioning of LUNs is provided in FIG. 9.Typically, an administrative request is initially made 142 to provide astorage space for a computer system 12. Conventionally, anadministrative application is provided to interact with the logicalvolume manager 62 to request 144 allocation of one or more data storageunits from the data storage systems 16. In response, the data storagesystems 16 provision, as required, one or more data storage units 146totaling the requested data storage capacity. The corresponding LUNidentifiers are then retrieved 148 by the logical volume manager 62 andthe creation of a corresponding data storage volume is completed 150.

The logical volume manager then initializes 152 the LUN controlstructures required for use of the data storage volume by a computersystem 12. In accordance with the present invention, the DSV metadatatables 42 are also created and initialized within the data storageunits. Specifically, the DSV metadata tables 42 are allocated within thecontrol structures of the LUN data storage spaces 40 and initializedwith the values of the LUN identifiers. A unique data storage volumeidentifier is generated and written as the DSV.ID values in each of theDSV metadata tables 42. A zero generation value is assigned as theDSV.Gen value. Each data storage unit is assigned a sequential DSU.Numwithin the data storage volume and, where used, the cardinality value ofthe data storage volume is written as the DSV.Card value in each of theDSV metadata tables 42.

Once the data storage volume is initialized, the logical volume manager62 will typically mark each of the individual data storage unitscomprising the data storage volume as active 156. In turn, the logicalvolume manager 62 will present the corresponding data storage volume asactive 158 to the associated filesystem layer 64, effectively enablinguse by the computer systems 12.

The processes 170 involved with the ongoing utilization of the presentinvention are shown in FIG. 10. Although subject to proprietary detailsas conventionally implemented by different commercial data storagesystem vendors, a replication process 172 is typically executed by thestorage system manager 24 independently on each of the data storagesystems 16. The replication process 172 may be instigated in response toan administrative request issued from the computer systems 12.Additionally, for the preferred embodiments of the present invention,the replication process 172 also executes autonomously in response to atimer event or at intervals determined preferably by a chronologicallyscheduled task, such as supported by a conventional cron-based daemonapplication. In response to an event occurrence, a data storage system16 instance will typically determine a set of local data storage unitsthat are enabled for replication 174. A typically proprietaryreplication operation is applied to create new point-in-time datastorage unit copies 176. As is conventional, the replicated data storageunits are assigned new LUN identifiers consistent with the SCSI protocolfor provisioning new LUNs.

The re-signaturing process 178 is initiated either administratively 180or in response to a timer event, preferably established as achronologically scheduled task invoked from a conventional cron-baseddaemon application or equivalent. The re-signaturing process 178 may beexecuted independently by any number of different computer systems 12.Since the re-signaturing operation is well-defined by the informationderived from the data storage units themselves, even overlappingexecution by multiple computer systems 12 is permitted.

In response to the initiating event 182, an executing instance of there-signaturing process 178 will first perform 184 a bus scan to identifydata storage units visible to the logical volume manager 62 responsiblefor the execution of the re-signaturing process 178. In a preferredembodiment of the present invention, the data storage units pendingre-signaturing are first identified by differences in the LUN.ID andDSU.ID of the data storage units as retrieved in connection with the busscan 184. To then re-signature these visible data storage units, the DSVmetadata tables 42 are updated to conform the DSU.ID value and incrementthe DSV.Gen value to reflect the generation of the point-in-timesnapshot 186. In an alternate embodiment of the present invention, wheremultiple LUN replications may occur before an execution of there-signaturing process 178, different generations of point-in-timesnapshots may be distinguished by a replication or other timestampdistinguishing the set creation of new point-in-time data storage unitcopies 176. Preferably, the DSV.Gen value is incremented to a value thatensures the tuple <DSV.ID, DSV.Gen> is unique. Finally, provided thatall members of a unique volume and generation set <DSV.ID, DSV.Gen> arevisible, a logical volume manager 62 can activate the corresponding datastorage volume 188. In accordance with the present invention, activation188 of a data storage volume can be performed by a logical storagemanager 62 as executed by any of the computer systems 12.

The principal steps of a preferred re-signaturing process 178 arepresented in pseudo-code in the following Tables I through III. Theinitial step, as represented by the exemplary pseudo-code presented inTable I, includes performing a bus scan to establish an input set Sidentifying all of the data storage units then visible to a particularlogical volume manager 62. Each data storage unit in the set S is thenexamined to determine if re-signaturing is required and thecorresponding DSV metadata table 42 updated, as appropriate. Eachexamination and update is preferably performed as an atomic operation.Consequently, this first step can and preferably is asynchronouslyexecuted without conflict by any number of different logical volumemanagers 62.

TABLE I Input S = {all DSUs visible to the logical volume manager }; foreach DSU d in S do {   reserve DSU d;   if (d.LUN.ID <> d.DSU.ID) {     d.DSV.Gen = d.DSV.Gen + 1;      d.DSU.ID = d.LUN.ID;      commitd.DSU.ID and d.DSV.Gen to DSU d;   }   release DSU d; }

In a second step, as represented by the exemplary pseudo-code presentedin Table II, LUNs corresponding to the same data storage volume are thenfurther updated to segregate the data storage units into setsdistinguished by generation.

TABLE II input S = { all DSUs visible to the logical volume manager };  for each DSU d in S, do {     if (d.LUN.ID == d.DSU.ID) {      S′= S −{d};      for each DSU d′ in S′, do       reserve DSU d′=;       if (d.DSV.ID == d′.DSV.ID and        d.DSV.Gen == d′.DSV.Gen and       d′.LUN.ID == d′.DSU.ID ) then {         d′.DSV.Gen = d′.DSV.Gen +1;         commit d′.DSV.Gen to DSU d′;        }       release DSU d′=;     }     }   }

At this point, the DSV metadata tables 42 have been updated to mutuallyassociate the data storage units belonging to the same generational datastorage volume. The pseudo-code in Table III outlines a data storagevolume discovery algorithm that can be subsequently executed by anylogical volume manager 62 to discover all data storage volumes thenavailable for use by any of the computer systems 12 at that point intime. This discovery algorithm will recognize the data storage unitsalready processed by the steps presented in Tables I and II.

TABLE III input S = { all DSUs visible to the logical volume manager };output V = { NULL }; // empty set to hold discovered DSVs A =Associative array indexed by the DSV primary key <DSV.ID, DSV.Gen>; foreach DSU d in S, do {   if (d.DSU.ID == d.LUN.ID) then {     A<d.DSV.ID,d.DSV.Gen> =     A<d.DSV.ID, d.DSV.Gen> U {d};   } } for each set ofDSUs s in A<i, j> {   print “Discovered DSV s”;   V = V ∪ s; }

At this point, the re-signatured data storage units are uniquelyidentifiable to the logical volume manager 62 as independent datastorage volumes, but also readily recognized as members of a consistentreplication series of data storage volumes. The integrity of the datastorage volumes, internally and as between the replicated and generationzero data storage volume is assured.

A subsequent rollback or remount request is handled as illustrated inthe process flow 190. In the event of a rollback request, theconventional steps 192 necessary to unmount a current, presumablygeneration zero data storage volume are performed. A target data storagevolume is then selected 194 for mounting. Optionally, the selectionprocess may invoke an iteration of the re-signaturing process 178 toensure that all currently replicated LUNs are re-signatured andavailable for mounting. The selected target data storage volume is thenmounted 196.

The process flow 190 can also be used to simply mount a priorpoint-in-time data storage volume for access by a computer system 12.The rollback preparation step 192 is not required. In the selection step194, an option is preferably offered and nominally accepted to furtherupdate the DSV metadata tables 42 of a selected point-in-time datastorage volume to assign a unique volume identifier. By furtherre-signaturing a data storage volume with a unique volume identifier,the logical storage manager 62 is preferably able to mount the datastorage volume with full data modification rights. Optionally, theassignment of a unique volume identifier is accompanied by resetting theDSV.Gen generation value.

Thus, a system and method providing for the autonomous re-signaturingdata storage volumes of automatically replicated LUNs has beendescribed. While the present invention has been described particularlywith reference to data storage units representing SAN virtualized LUNs,the present invention is equally applicable to data storage systems thatimplement data storage unit directly as physical LUNs. In addition, thepresent invention can be applied to data storage systems that virtualizecomplete data storage volumes, rather than just data storage units. Inthis case, the re-signaturing process executes against the singlecombined data storage entity.

In view of the above description of the preferred embodiments of thepresent invention, many modifications and variations of the disclosedembodiments will be readily appreciated by those of skill in the art. Itis therefore to be understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed above.

1. A computer implemented method of autonomously replicating datastorage volumes within a storage area network, wherein storage areanetwork devices provide for, at programmed intervals, replication ofselected data storage units hosted by the storage area network devices,said method comprising the steps of: scanning, by a computing device, atpredetermined intervals, a collection of data storage units visible to adata storage volume manager to identify a set of data storage unitsinternally related as a data storage volume, wherein said step ofscanning includes retrieving predetermined metadata signature valuespersistently stored within the respective data storage units of saidcollection, said predetermined metadata signature values including adata storage unit identifier, a logical unit identifier, and a datastorage volume identifier; identifying, by the computing device, datastorage units that have been replicated as those having the logical unitidentifier different from the data storage unit identifier, whereinreplication of data storage units provides each replicated data storageunit with a new logical unit identifier while the data storage unitidentifier is not changed during replication; determining, by thecomputing device, for the replicated data storage units, revisedmetadata signature values globally distinct where the data storage unitidentifier is changed to be equal to the logical unit identifier; andre-signaturing, by the computing device, said replicated data storageunits with said revised metadata signature values, whereby said datastorage volume represented by said replicated data storage units can beactivated concurrent with the data storage volume of which saidreplicated data storage units were replicated due to the change of thedata storage unit identifier to be different from data storageidentifiers of other data storage units.
 2. The computer implementedmethod of claim 1 wherein said predetermined metadata signature valuesinclude ordering identifiers that are mutually unique within said set ofdata storage units and wherein said step of identifying further includesdetecting a complete set of said ordering identifiers among saidreplicated data storage units.
 3. The computer implemented method ofclaim 2 wherein said predetermined metadata signature values includes acommon cardinality value identifying the number of data storage unitsincluded within said data storage volume.
 4. The computer implementedmethod of claim 3 wherein said step of scanning further includescontinue scanning data storage units until all of the data storage unitsof said set of data storage units are accessible by said data storagevolume manager.
 5. The computer implemented method of claim 1 whereinsaid predetermined metadata signature values further include ageneration identifier, wherein first and second instances of said set ofdata storage units are replicated from a current set of data storageunits, and wherein said step of determining said revised metadatasignature values provides for selecting a unique value for saidgeneration identifier for each of said first and second instances ofsaid set of data storage units.
 6. The computer implemented method ofclaim 1 wherein said step of determining further includes selecting aglobally unique value for said data storage volume identifier for saidreplicated data storage units.
 7. A computer system providing for theautonomous administration of automatically replicated data storage unitsas globally consistent, generationally distinct data storage volumeswithin a distributed data storage system, said computer systemcomprising: a host computer system coupleable through a communicationsnetwork to one or more storage area network devices, wherein eachstorage area network device provides for the storage of data storageunits and includes a data store controller providing for the replicationof data storage units at respective first predetermined intervals; and adata storage volume manager executed by said host computer system, saiddata storage volume manager providing for, at second predeterminedintervals, identification of a replication instance of a data storagevolume, which includes a plurality of data storage units stored by saidone or more storage area network devices, by examination ofpredetermined metadata stored respectively by said plurality of datastorage units, said predetermined metadata including a logical unitidentifier, a data storage volume identifier and a data storage unitidentifier, the replication instance having a new logical unitidentifier while the data storage unit identifier is not changed duringreplication, said data storage volume manager further providing for there-signaturing of said replication instance by re-writing saidpredetermined metadata of the data storage units of the replicationinstance with a common data storage volume identifier and with datastorage unit identifiers matching respectively said logical unitidentifiers, wherein said common data storage volume identifier isglobally unique with respect to other data storage volumes stored bysaid one or more storage area network devices.
 8. The computer system ofclaim 7 wherein said data storage volume manager provides for theidentification of first and second replication instances and whereinsaid data storage volume manager, in re-writing said predeterminedmetadata, further provides for the writing of a generation identifier aspart of said predetermined metadata, said generation identifier beingunique as between said first and second replication instances of saidpredetermined data storage volume.
 9. The computer system of claim 8wherein said data storage volume manager operates to scan said storagearea network devices to retrieve data storage unit metadata, whereinsaid replication instance is identified by correspondence with a set ofmetadata where the value of said logical unit identifier differs fromthe value of said storage unit identifier, where said data storagevolume identifier is shared by said set of metadata, and wherein thecardinality of said set of metadata is equal to the shared value of acardinality identifier stored by each metadata of said set of metadata.10. The computer system of claim 9 wherein said storage area networkdevices provide for the storage of multiple copies of said data storagevolume, each of said multiple copies identified by a common data storagevolume identifier, wherein said data storage volume manager furtheroperates to write a generation identifier into each metadata of said setof metadata, wherein said generation identifier serves to uniquelyidentify said replication instance within the context of said multiplecopies.
 11. A computer system operable within a distributed computingenvironment wherein said computer system is one of a like plurality ofcomputer systems mutually accessing data storage units hosted by storagearea network (SAN) storage systems present within said distributedcomputing environment, wherein said computer system comprises: a hostplatform coupleable through a communications network to one or more SANstorage systems, wherein said SAN storage systems provide access torespective collections of data storage units, and wherein said datastorage units are subject to automatic replication within said SANstorage systems at defined checkpoint intervals implementedindependently by said SAN storage systems, wherein said data storageunits are identified by respective unique logical unit (LUN)identifiers, wherein said data storage units include data storage volumemetadata tables containing a data storage volume identifier, the LUNidentifier, and a data storage unit identifier; an operating system,including a file system executable by said host platform; a logicalvolume manager, executable in conjunction with said file system, thatenables access by said file system to a data storage volume, saidlogical volume manager performing access translation between said datastorage volume and a volume related plurality of data storage units; anda re-signaturing manager, executable by said host platform inconjunction with said logical volume manager, said re-signaturingmanager providing for the identification of replicated volume relatedsets of data storage units from among said respective collections ofdata storage units based on a comparison of said LUN identifiers, datastorage volume identifiers, and data storage unit identifiers, whereinthe identification of replicated volume related sets of data storageunits is determined based on replicated volume related sets of datastorage units being provided a new logical unit identifier while thedata storage unit identifier is not changed during replication, saidre-signaturing manager further providing for the re-writing of said datastorage volume metadata tables of a predetermined volume related set ofdata storage units to uniquely define a predetermined data storagevolume.
 12. The computer system of claim 11 wherein said re-signaturingmanager is executable at intervals independent of the correspondingexecution intervals for said re-signaturing managers of others of saidlike plurality of computer systems.
 13. The computer system of claim 12wherein said re-signaturing manager defers re-writing of said datastorage volume metadata tables of said predetermined volume related setof data storage units until said re-signaturing manager can re-write allof the data storage units of said predetermined volume related set ofdata storage units.
 14. The computer system of claim 13 wherein saiddata storage volume metadata tables include a cardinality valuedeterminative of the number of data storage units present within acorresponding volume related set of said data storage units.
 15. Thecomputer system of claim 14 wherein identification of a replicatedvolume related set of data storage units is determined by a non-identitybetween the LUN identifier and data storage unit identifier for a givendata storage unit and an identity of data storage volume identifiersamong said replicated volume related set of data storage units.
 16. Thecomputer system of claim 15 wherein re-writing of said respective datastorage volume metadata tables of said predetermined volume related setof data storage units establishes an identity between the LUN identifierand data storage unit identifier for each of the data storage unitswithin said replicated volume related set of data storage units andestablishes a unique data storage volume identifier value for the datastorage units within said replicated volume related set of data storageunits, wherein said unique data storage volume identifier is unique withrespect to other data storage volume identifiers present within saidrespective collections of data storage units.
 17. The computer system ofclaim 12 wherein a subset of said predetermined volume related set ofdata storage units are visible to said re-signaturing manager andwherein said re-signaturing manager re-writes said data storage volumemetadata tables of said subset of said predetermined volume related setof data storage units, whereby said data storage volume metadata tablesof said subset of said predetermined volume related set of data storageunits are collectively re-written by said re-signaturing managers ofsaid like plurality of computer systems.
 18. The computer system ofclaim 17 wherein identification of a replicated volume related set ofdata storage units is determined by a non-identity between the LUNidentifier and data storage unit identifier for a given data storageunit and an identity of data storage volume identifiers among saidreplicated volume related set of data storage units.
 19. The computersystem of claim 18 wherein re-writing of said respective data storagevolume metadata tables of said predetermined volume related set of datastorage units establishes an identity between the LUN identifier anddata storage unit identifier for each of the data storage units withinsaid replicated volume related set of data storage units and establishesa unique data storage volume identifier value for the data storage unitswithin said replicated volume related set of data storage units, whereinsaid unique data storage volume identifier is unique with respect toother data storage volume identifiers present within said respectivecollections of data storage units.